Al-Mansur al-Qasim (November 13, 1559 – February 19, 1620), with the cognomen al-Kabir (the Great), was an Imam of Yemen, who commenced the struggle to liberate Yemen from the Ottoman occupiers. He was the founder of a Zaidi kingdom that endured, under many vicissitudes, until 1962.
Al-Qasim bin Muhammad was a fourteenth-generation descendant of the imam ad-Da'i Yusuf (d. 1012). His father supported the imam al-Mutahhar (d. 1572), who fought the encroaching Ottomans with partial success but who was finally defeated in 1569-1570. Al-Qasim was a religious teacher at the Dawud mosque in San'a at a time when the Ottoman grip on Yemen was severely felt. The Turks promoted the Sunni legal tradition of Hanafi, at the expense of the Zaydiyyah which dominated in the highlands of Yemen. One of al-Qasim's pupils suggested him to claim the Zaidi imamate, which he first declined. The suspicions of the Turks were however raised, and al-Qasim fled San'a, finally setting forth his claim (da'wah) to the imamate in Hajur in the north-west in September 1597.
In his theological thinking al-Mansur al-Qasim upheld the Shia foundations of Zaydiyyah by stressing the eighth-century Jarudi position to the imamate which considered the first two caliphs as usurpers. He emphasized the differences between Zaydiyyah and the Mu'tazila school of theology, which laid stress on reason and rational thought, whereas some previous Yemeni imams had noted the similarities. He argued that the early imams had limited their prescriptions to what could be traced to reason, the unambiguous Qur'an text, and the generally accepted Sunnah. These imams, he argued, did not follow the Mu'tazila in their speculations and fantasies. Al-Mansur al-Qasim was also hostile to Sufism, one reason being that the Sufi orders supported the Ottoman occupants. He branded the Sufis as heretics and likened them to Ismailites, traditional enemies of the Zaidis.